Fix multiple lines bugs

[emeitch]

I use DB table and column comments by multiple lines. But migration_comments annotation feature brokes annotations when there are multiple lines.

e.x.)

DB a table users comment:

this
table for
users

migration_comments annotation:

# Table name: users # this
table for
users
#
#

↑ it's broken as comments.

I want the following annotation.

# Table name: users # this
#                   # table for
#                   # users
#
#

[emeitch]

The Travis CI failed. But, it is not this PR effects.